The GAA triplet-repeat is unstable in the context of the human FXN locus and displays age-dependent expansions in cerebellum and DRG in a transgenic mouse model
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Friedreich ataxia (FRDA) is caused by homozygosity for FXN alleles containing an expanded GAA triplet-repeat (GAA-TR) sequence. Patients have progressive neurodegeneration of the dorsal root ganglia (DRG) and in later stages the cerebellum may be involved. The expanded GAA-TR sequence is unstable in somatic cells in vivo, and although the mechanism of instability remains unknown, we hypothesized that age-dependent and tissue-specific somatic instability may be a determinant of the progressive pathology involving DRG and cerebellum. We show that transgenic mice containing the expanded GAA-TR sequence (190 or 82 triplets) in the context of the human FXN locus show tissue-specific and age-dependent somatic instability that is compatible with this hypothesis. Small pool PCR analysis, which allows quantitative analysis of repeat instability by assaying individual transgenes in vivo, showed age-dependent expansions specifically in the cerebellum and DRG. The (GAA)190 allele showed some instability by 2 months, progressed at about 0.3–0.4 triplets per week, resulting in a significant number of expansions by 12 months. Repeat length was found to determine the age of onset of somatic instability, and the rate and magnitude of mutation. Given the low level of cerebellar instability seen by others in multiple transgenic mice with expanded CAG/CTG repeats, our data indicate that somatic instability of the GAA-TR sequence is likely mediated by unique tissue-specific factors. This mouse model will serve as a useful tool to delineate the mechanism(s) of disease-specific somatic instability in FRDA.
This research was made possible in part by grants from the National Institutes of Health (NINDS), Muscular Dystrophy Association, OCAST, and FARA to S.I.B. M.P. acknowledges support form The Wellcome Trust. I. D-B. is supported by a postdoctoral fellowship from the National Ataxia Foundation.
- Bidichandani SI, Ashizawa T, Friedreich Ataxia (2006) In: GeneReviews: genetic disease online reviews at GeneTests–GeneClinics (updated). Copyright, University of Washington, Seattle. Available at http://www.geneclinics.org/
- Brook JD, McCurrach ME, Harley HG, Buckler AJ, Church D, Aburatani H, Hunter K, Stanton VP, Thirion J-P, Hudson T, et al (1992) Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3′ end of a transcript encoding a protein kinase family member. Cell 68:799–808PubMedCrossRefGoogle Scholar
- van Den Broek WJ, Nelen MR, Wansink DG, Coerwinkel MM, te Riele H, Groenen PJ, Wieringa B (2002) Somatic expansion behaviour of the (CTG)(n) repeat in myotonic dystrophy knock-in mice is differentially affected by Msh3 and Msh6 mismatch-repair proteins. Hum Mol Genet 11:191–198PubMedCrossRefGoogle Scholar
- Goldberg YP, Kalchman MA, Metzler M, Nasir J, Zeisler J, Graham R, Koide HB, O’Kusky J, Sharp AH, Ross CA, Jirik F, Hayden MR (1996) Absence of the disease phenotype and intergenerational stability of the CAG repeat in transgenic mice expressing the human Huntington’s disease transcript. Hum Mol Genet 5:177–185PubMedCrossRefGoogle Scholar
- Gomes-Pereira M, Bidichandani SI, Monckton DG (2004a) Analysis of unstable triplet repeats using small-pool polymerase chain reaction. Methods Mol Biol 277:61–76Google Scholar
- Lia AS, Seznec H, Hofmann Radvanyi H, Radvanyi F, Duros C, Saquet C, Blanche M, Junien C, Gourdon G (1998) Somatic instability of the CTG repeat in mice transgenic for the myotonic dystrophy region is age dependent but not correlated to the relative intertissue transcription levels and proliferative capacities. Hum Mol Genet 7:1285–1291PubMedCrossRefGoogle Scholar
- Pandolfo M, Koenig M (1998) Clinical and pathological aspects of Freidreich ataxia. In: Wells R, Warren S, Sarmiento M (eds) Genetic instabilities and hereditary neurological diseases, Academic, New York, pp 373–398Google Scholar
- Seznec H, Lia-Baldini AS, Duros C, Fouquet C, Lacroix C, Hofmann-Radvanyi H, Junien C, Gourdon G (2000) Transgenic mice carrying large human genomic sequences with expanded CTG repeat mimic closely the DM CTG repeat intergenerational and somatic instability. Hum Mol Genet 9:1185–1194PubMedCrossRefGoogle Scholar
- The Huntington’s Disease Collaborative Research Group (1993) A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington’s disease chromosomes. Cell 72:971–983Google Scholar